Electromagnetic current control for a hollow cathode



21g-121 SR www mm@ Melzo@ ma 3,377,506.

April 9, 196s C. M. ANAS ETAL 3,377,506

ELECTROMAGNETIC CURRENT CONTROL FOR A HOLLOW CATHODE Filed March 50,1966 2 Sheets-Sham l April 9, 1968 c. M. BANAS TAL 3,377,506

ELECTROMAGNETIO CURRENT CONTROL FOR A HOLLOW CATHODE Filed March 50,1966 l 2 Sheets-Sheet 2 a [ffii United States Patent O ABSTRACT F THEDISCLOSURE A magnetic current control device is described for a hollowcathode structure which operates in a glow dis charge and produces acollimated beam of electrons from the volume enclosed by the cathodestructure. The number of electrons produced in the beam is controlled bysubjecting the plasma within the hollow cathode structure to a magneticfield and varying the intensity of the field to produce the desired beamcurrent.

This invention relates to a device for controlling the current in a beamof charged particles emanating from an aperture in a hollow electrode.More specifically, it relates to a device for controlling the magnitudeof the current in a beam of charged particles produced from a plasma ina hollow electrode with a magnetic lield acting on the plasma.

Conventional means for producing electron beams inn volve the liberationof electrons from the surface of a heated cathode by thermionicemission. Recently, electron beams have been produced from an aperturedcathode as a result of the volume production of electrons within ahollow chamber enclosed by the cathode. In glow discharge devicesvarious operating modes may be end countered if one, for instance,varies the potential diiference between the cathode Vand the anode andgas density of the environment One of these modes produces a welldelined electron beam which may predictably and advan tageously be usedto work materialsu These apertured hollow cathodes have hollow chambersand are fabricated from a wire mesh or solid metal with a singleaperture in one end. When the cathode is subjected to a high negativepotential with respect to an anode and with the proper cathode geometryand pressure level in the hollow chamber, a Well-defined pencil-formbeam of high current density, high energy electrons emanates from theaperture. An example of the versatility of the configurations possiblewith the apertured hollow cathode may be found in the copendingapplication Ser. No. 417,399, filed Dec. l0, 1964, entitled Annular Holnlow Cathode Discharge Apparatus by Fernand I. Fer-1 reira, and assignedto the same assignee.

The aperture size has a strong influence on the operatd ing mode and thecharacteristics of the beam. Its dimensions must be approximately equalor larger than the cathode fall. This dimensional requirement will varywith different gases and gas densities.

A large fraction of the electrons for the beam are obtained from aplasma generated by an intense discharge within the chamber enclosedbythe cathode. Various controls have been proposed to vary the currentobtainable from the hollow cathode chamber and to maintain the electronbeam current within precisely controlled limits. One such control, forinstance, would involve varying the anode-to-cathode potential tomaintain the desired current. 'One disadvantage of varying thispotential is that the focal length of an electron lens depends upon itand compensation would have to lbe made in order to maintain a fixedbeam spot size on a workpiece. This is especially so 3,377,506 PatentedApr. 9, 1968 ICC at the low pressure end of the cathode operating regimein which a small variation in the potential may even result inextinguishment of the discharge.

Another approach suggested to obtain the current control has been tovary the gas pressure in the hollow cathode. This approach involvescomplicated electromechanical devices. Generally, the mass liowcharacteristic of a diffusion pump slopes downward with increasingpressures. With operating pressures, for instance, within the range fromone to microns an increase in the gas ,tiow also causes an increase inpressure due to a decrease in the pumping speed and unstable operationresults. Hence, a complicated throttling device on the diifusion pump ora mechanical pump having a high-speed response is needed. In operationssuch as welding, a significant amount of material is evaporated and thepressure of the environment is affected. Hence, a pumping system with afast response is needed to control the gas pressure and maintain thebeam current within the desired limits.

Still another approach involves the use of a control grid which operatesadjacent to the emitting surfaces of the hollow cathode and suppresseselectron ow from the cathode to the plasma when a suitable negativepotential is applied. The disadvantage of this type of current controlis that contact with the high density plasma leads to disintegration ofthe control grid. v

It is therefore an object of this invention to provide a current controlfor a beam of charged particles emanating from a hollow electrode.

It is a further object of this invention to provide a device forcontrolling the magnitude of the current in a beam of charged particlesproduced from a plasma in a hollow electrode with a magnetic lieldacting on the plasman It is still another object of this invention toprovide a magnetic current control for a iiow discharge hollow cathodeoperating in the electron beam mode.

These and other objects will becomev more readily apparent upon a reviewof the drawings and specification wherein:

FIGURE l shows a cross-sectional view of the current control.

FIGURE 2 shows the linearity of the current control device at differentanode-to-cathode potentials.

FIGURE 3 shows a block diagram of an automatic control system which isused to maintain the electron beam current from the hollow cathode at apreselected reference value.

FIGURE 4 shows the eifectiveness of this current control with varyingpressure conditions.

In this invention, current control of a hollow cathode operating in theelectron beam generating mode is obtained by applying a magnetic fieldto the chamber 15 enclosed by the cathode wherein the plasma producingthe beam of electrons is active. In FIGURE 1 a cathode 10, shown insectional form, comprises a cylindrical hollow member which is Amade ofa non-magnetic material such as tantalum. The back wall 11 and theperipheral wall 13 define the cylindrical chamber 15. In addition, theperipheral wall is shaped to form an aperture 12. substantiallyconcentric therewith and opposite the back wall. The cathode 10 iselectrically connected to a highvolt age supply 18 via a supportingconductor 14 and a lead 16. The aperture dimensions are approximately 1Aythat of the face in which it appears. Surrounding the cathode 10 is adischarge suppressor shield 20 which extends back to the wall of thechamber 22 and surrounds the conductor 14 as well as the nonemittingsurfaces of the cathode 10. A typical hollow cathode cylindricalstructure having an insulator shield is described in the copendingapplication tiled by Conrad M. Banas and Clyde O. Brown 3 entitledInsulator Shielded Cathode, Ser. No. 506,237, filed Now 3, 1965, andassigned to the same assignee.

Surrounding and coaxial with the cathode and the shield 20 is asolenoid-type coil 24 located generally at the center of the cathode andprovides a magnetic field that is substantially parallel to thecylindrical axis of the cathode 10. The coil 24 is so located withrrespect to the cathode that its magnetic field acts predominantly on theplasma generated within the chamber 15. The field may also diverge fromthe cylindrical axis in the manner of solenoids. The current for thecontrol coil 24 is provided by a control power supply 26 that is locatedoutside of the chamber 22. Spaced from the cathode aperture 12 is anadditional magnetic coil 30 designed to further focus the beam emanatingfrom the hollow cathode 10 at the workpiece 32., The workpiece in thisinstance operates as the anode but as described in thepreviously-mentioned patent application to Ferreira, the anode may belocated anywhere within the chamber 22 provided it is sufficientlydistant from the cathode 10 The current for the focusing coil 30 issupplied from an external supply 34. Interposed between the control coil24 and the workpiece 32 is a shield 28 to protect the control coil fromthe heat generated by the working of the electron beam on the workpiece$2.` This shield 28 may be of the same material as shield 20.

The control coil 24 produces an electromagnetic field that acts throughthe shield 20 and the wall of the cathode 10 on the plasma enclosed -bythe hollow cathode 10. Contrary to what prior experience in plasmaphysics might lead one to expect, application of a magnetic field withinthe hollow cathode increases the current from the hollow cathode throughthe aperture 12., As is shown in FIGURE 2, in the absence of any currentflowing through the control coil 24, the hollow cathode operates in itsnormal fashion producing various currents for different anode-to-cathodepotential differences. As the magnetic field of the control coil 24 isincreased, the electron beam current increases as well. The effect ofthe magnetic field on the discharge is similar to that obtained byvariation in chamber pressure.. Hence, a substantial simplification ofcurrent control is possible with change in operating pressures. Suchpressure variations arise, for instance, during welding or fromirregularities in the pumping system and may be readily compensated for.This control is simple, since, as can be seen from FIGURE 2, the currentvaries practically linearly with the strength of the magnetic fieldapplied by the control coil 24 The advantage of this invention becomesclearly evident from FIGURE 4 In this figure the operating lines of atypical hollow cathode are shown for different magnetic field strengthsoAlthough current values indicate the discharge current, its valuesreflect the beam current since an advantage of this type of control isits preservation of the efficiency. Thus for example, if a hollowcathode operates at 20 kilovolts and the field strength from controlcoil 24 as fixed by the field references 58 is 20 gauss, the current inthe electron beam is about 190 milliamperes with a pressure of 8 microns(.008 mm.) of mercury. Under these conditions the cathode operates atpoint C in FIGURE 4.. If the pressure in the chamber rises to 9 microns,the operating point of the device will shift to point A along the 20gauss line and to compensate for this increase in current withoutchanging the pressure the control current. through coil 24 is reduced tolower the magnetic field strength to that for curve B1 where thecorresponding current level is again 190 milliarnperesu Similarly,corrections are made for reductions in pressure levels but now themagnetic field intensity from the control coil 24 is increased so thatoperation of the cathode proceeds from C to D to E.

The excursion of the cathode from desired operating current has beenexaggerated in FIGURE 4 for clarityD Actually, with a quickly responsivecontrol feedback circuit of the type shown in FIGURE 3, essentiallyconstant current may be maintained.. A very small change in. current asa result of a change in chamber pressure is observed and the current inthe control coil 24 is immediately changed to compensate for the change.As a result of a one micron increase in the chamber pressure, the hollowcathode will reach the new operating point B essentially along the lineCB with only very small excursions.

The curve of FIGURE 4 .is for a constant voltage source. Athree-dimensional model could be envisioned if voltage is also avariable. The design operating conditions are in general a function ofvarious requirements such as imposed by the material, stability of thebeam, type of work to be accomplished, etc.,

The hollow electrode may be used to generate a plasma therein from whicha beam of ions is extracted. This invention although described inrelation with an electron beam is also applicable to beams of ionparticles and control the current magnitude of the ion beam. Higherfield strengths are needed to control the heavier ion particles and thisis simply provided by increasing the coupling between the coil 24 andthe cavity or cham-ber 15G An automatic control as shown in FIGURE 3operates as follows. The resistor 50 is shown in series with the highvoltage supplier 18 so that the voltage developed across it will have adirect relationship with the current emanating from the hollow cathode10. This resistor may actually ybe the current sensing element of theelectron beam power supply. The voltage developed across resistor 50 isfed to a difference amplifier 52 to generate an output signal thatrefiects the difference 4between the current sensed by the resistor 50and a preselected reference value set by the circuitry 54. The referencevalue indicates the desired beam current from the hollow cathode, Theamplifier 52 output signal is then applied through a magnetic amplifier56 to the control coil 24 to provide the desired magnetic fieldstrength. The initial operating point of the magnetic amplifier 56corresponding to point C is determined by the field reference circuit58. The error signal from differential amplifier 52 is superimposed onthe field reference signal, By selecting the polarity of the currentsensed by the resistor 50 on the difference amplifier 52 current may beappropriately increased or decreased and a fast and reliable currentcontrol for a hollow cathode discharge may be provided by driving theerror signal to a minimum.

It is to be understood that the invention is not limited to the specificembodiments herein illustrated and described but may be used in otherways without departure from its spirit as defined by the followingclaims.

We claim:

1. A device for controlling the flow of charged particles from a hollowcathode operating at a high potential difference with respect to ananode in a gaseous environment comprising:

a hollow cathode structure having a chamber evacuated to a predeterminedgaseous pressure,

said chamber being provided with an aperture,

means establishing a glow discharge within said chamber and producing abeam of charged particles from said aperture, and

an electromagnetic coil adjacent the chamber for generating a magneticeld within said chamber and vary ing the number of charged particles inthe beam Vfrom said cathode chamber aperture as a function of thestrength of the magnetic field 2. A device for controlling the flow ofelectrons from a hollow cathode operating at a high negative potentialdifference with respect to an anode in a gaseous environment comprising:

a hollow cathode structure having a chamber evacuated to a predeterminedgaseous pressure,

means producing a glow dischar'ge within said chamber,

said chamber being provided with an aperture having dimensionalcharacteristics for establishing a beam of electrons therefrom, and

means for generating a magnetic field within said cham-1 ber, saidmagnetic field increasing the electron current in the beam with anincrease in magnetic field strength and decreasing the electron currentin the beam with a decrease in magnetic eld strength.

3. A device as recited in claim 2 wherein the hollow cathode structurecomprises:

a cylindrical chamber having nonmagnetic walls and provided with saidaperture in one end, said aperture being substantially coaxial with thecylindrical axis of the chamber,

and where said magnetic field generating means comprises.

a solenoid externally adjacent to the chamber and substantially coaxialtherewith for producing a magnetic field in the chamber that is substanmtially parallel with said cylindrical axis.

4. A device as recited in claim 3 and further comprislng:

a nonmagnetic glow discharge suppressor shield intenl posed between thesolenoid and the chamber,

where the shield is selectively spaced from and substantially enclosesthe cylindrical wall of said chamu ber to suppress the glow dischargetherebetween.

5. A device as recited in claim 4 and `further comprislng:

a heat shield interposed between the solenoid and the lbeam ofelectrons.

6. A device for controlling the magnitude of the current in a beam ofelectrons produced by a hollow cathode operating in a glow discharge toperform work on workn pieces comprising:

a hollow cathode structure,

said cathode structure having a back wall,

a peripheral wall extending away from said back wall to define a cavityof sufficient size to establish said glow discharge therein,

said peripheral wall further forming an aperture from which the beam ofelectrons emerges,

an electromagnetic coil wrapped about said peripheral wall to produce amagnetic eld in said cavity, and

means supplying current through said electromagnetic coil to vary thestrength of the magnetic field and control the magnitude of the currentin the beam.

7. A device as recited in claim 6 and further comprisa nonmagnetic glowdischarge suppressor shield interposed .between the electromagnetic coiland said peripheral wall,

said shield being selectively spaced from and substan-x tially enclosingthe peripheral wall to suppress the glow discharge there-between, and

a heat shield extending outwardly from said peripheral wall in betweenthe beam of electrons and said electromagnetic coil toshield the coilfrom the heating effects of said beam of electrons upon the workpieces.

8. A device as recited in claim 6 and further comprising:

means for generating a first signal indicative of the actual current inthe beam,

means for generating a second reference signal indicative of the desiredcurrent in the beam, and

means responsive to said first and second signals for producing an errorsignal and applying said error signal to said electromagnetic coilcurrent supply means with a polarity to vary the intensity of themagnetic field and control the current in the beam to the desired value.

9. A device as recited in claim 6 wherein the magnetic field in saidcavity is oriented substantially transverse to the aperture.

10. A device as recited in claim 9 wherein said peripheral wallcomprises:

a single cylindrical wall, and

wherein said magnetic coil is a solenoid to produce a magnetic fieldsubstantially coaxial with said single cylindrical wall.

11. A device as 4recited in claim 2 wherein said magnetic fieldgenerating means comprises an electromagnetic coil adjacent the cathodechamber and varying the numn1 ber of electrons in the beam from saidcathode chamber aperture as a function of the strength of the magneticfield.

References Cited UNITED STATES PATENTS 1,954,025 4/1934 Reynolds 313-84X 2,940,010 6/1960 Kenny 315-107 2,945,160 7/1960 Burk N 315-107 X3,152,238 1-0/1964 Anderson BSO- 49.5 X 3,243,570 3/1966 Boring 219-1213,320,475 5/1967 Boring u.. 315-108 JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, `Assistant Examiner.

